The invention relates to a process for the production of a metal alloy material capable of thixotropic forming.
The forming of metal alloys in the semi-solid state by means of thixocasting, thixoforging or thixopressure injection is gaining significance as an alternative to the classic methods for producing formed pieces by means of casting, forging and pressure injection. Thus, it is now possible to start with a material in the semi-liquid/semi-solid statexe2x80x94hereunder designated as semi-solidxe2x80x94to manufacture cast or forged structural components that meet high quality demands. Particularly when it comes to the production of heavy-duty, lightweight metal formed pieces with a complex geometry, forming in the semi-solid state offers great economic advantages. Thus, for example, the forming of aluminum or magnesium alloys in the semi-solid state is a hybrid process that combines the great design freedom and manufacturing speed of die casting processes with the quality advantages of forging processes.
The prerequisite for successful production by forming a material such as a metal alloy, in the semi-solid state is a special thixotropic behavior on the part of the material whereby the use of the term xe2x80x9cthixotropyxe2x80x9d refers to a thixotropic behavior in which mechanical stress due to shear stress leads to a substantial decrease in the material""s viscosity. It should be kept in mind that the viscosity under load changes by several orders of magnitude. Thus, for example, when a thixotropic metal alloy is in the unstressed state, its viscosity is about 106 to 109 Pas, which corresponds to the properties of a solid, whereas under shear stress, the viscosity drops to values of about 1 Pas, which corresponds to a viscosity between that of honey (10 Pas) and that of olive oil (10xe2x88x921 Pas).
It is known that, when a thixotropic material is in the unstressed state, the geometrical configuration of the solid phase is characterized by coherent grain groupings, which form a spatial skeleton When a shear stress is applied, these superstructures are broken apart, giving rise to a flowable suspension consisting of solid particles in a liquid matrix phase, hereunder designated as a xe2x80x9csolid-liquid suspensionxe2x80x9d. Accordingly, the semi-solid state of a material is a necessary but not yet sufficient condition for thixotropic behavior. On the contrary, the decisive aspect is a special configuration of the microstructure in which the above-mentioned spatial skeleton can be broken apart under shear stress. This condition cannot be met by all materials, first of all because the melt interval has to be sufficiently wide and secondly, because a special pretreatment is needed so that the structure of the solid phase does not become dendritic but rather globular.
The formation of a thixotropic fine structure is described, among other places, in EP 0090253 A, EP 0554808 A, EP 0745694 A, EP 0765945 A and EP 0792380 B1. A distinction is made essentially between the two process variants of conventional thixocasting (CTC) and new rheocasting (NRC). In the CTC process, a material that is usually made by means of stirred strand casting is inductively heated in portioned sections in the semi-solid state and subsequently, in a die casting machine, is transformed into a solid-liquid suspension that is pressure injected into a mold. With the NRC process, the globular material is made by a controlled cooling off of a melt in the semi-solid state that has been metered into steel crucibles.
Regardless of whether the semi-solid state of a material is achieved by heating a solid phase as is done in the CTC process or by cooling off the melt as is done in the NRC process, a decisive criterion as to whether a material can be transformed into a low-viscosity, solid-liquid suspension is the already mentioned globular structural evolution. The latter can be described essentially by four structural parameters, whereby it is advantageous to use the solid phase fraction, the form factor of the solid phase, the grain size of the solid phase and the degree of skeletization. Limit values for said structural parameters are only partially known from the state of the art.
EP 0554808 A, describes a process of the generic type for the production of a material made of a metal-alloy for a subsequent forming of the material in the semi-solid state. According to this teaching, the metal alloy is brought to a starting temperature that is above the liquidus and then a grain refiner is added to the melt thus formed. Subsequently, the metal alloy is cooled off to any temperature below the solidus and material thus formed is kept in the solid state essentially for any desired time. Finally, the material is brought into the semi-solid state by being heated up to a holding temperature that lies between the solidus and the liquidus, and is kept there for a holding time of less than 15 minutes. The forming of the material in the semi-solid state absolutely has to be carried out within the less than 15 minute holding time.
A drawback of such a process is that, since the holding time is limited to less than 15 minutes, the materials made by the process are not suitable for use in conventional forming installations. Consequently, processing by means of thixocasting, thixoforging or thixopressure injection of the materials made by means of the known process calls for the special production installations capable of ensuring that the forming is carried out within the processing window that is limited to less than 15 minutes. Another disadvantage of the process lies in the fact that the material first has to be cooled off from the molten state to the solid state and only then can it be brought into the semi-solid state for subsequent forming. This interim solidification is extremely undesirable, especially for an automated production and forming process.
An objective of the present invention is to provide an improved thixotropic process
In the process according to the invention for the production of a material made of a metal alloy for subsequent forming of the material in the semi-solid state, the metal alloy is brought to a starting temperature that is above the liquidus and then an additive is added which is capable of reducing an interfacial surface energy between the solid phase and the liquid phase after the metal alloy has been mixed with the additive and transformed into the semi-solid state. The volume fraction of the additive is selected such that, in the semi-solid material at a solid phase fraction of 25% to 85%, the grain size and the degree of skeletization during a holding time of more than 15 minutes both remain essentially constant in order to retain the formability of a suspension.
Since the grain size and the degree of skeletization remain essentially constant during a holding time of more than 15 minutes, a phlegmatization of the semi-solid material is achieved which allows a more advantageous production from both economic and environmental standpoints. The prolongation of the processing window leads to a reduction in rejects that are inevitably created with the prior art process whenever the thixotropic properties of the material are lost because the holding time is too long. Moreover, when the process according to the invention is used, thanks to the phlegmatization that can be achieved, the transformation of the material into the semi-solid state for the subsequent forming can be carried out directly from the melt, i.e. an interim solidification of the material is not necessary. In this manner, the acquisition of costly special production installations can be avoided or at least limited, and the ability arises for a far-reaching process integration of material production and subsequent forming. Moreover, the process sequence can be largely homogenized, even in existing production installations, as a result of the reduced structural sensitivity. If storage of the material is desired, it can be cooled off to a storage temperature that lies below the solidus and restored to the semi-solid state just before forming, without the advantageous phlegmatization being lost.
Using the material made with the process according to the invention, structural components can be made by a subsequent forming procedure that exhibit a good combination of strength and toughness, that can also be heat-treated and welded, and that are pressure-proof and relatively inexpensive.
The process can be used with various types of metal alloys. In a preferred embodiment, the metal alloy contains aluminum as the main component and barium is used as the additive, whereby the weight fraction of the barium may be 0.1% to 0.8% of the material. In view of the enormous importance of aluminum structural components, the advantages of such an embodiment is clear.
Especially good results are achieved when a dispersoid-forming element is added to the metal alloy in order to promote the formation of grains having a small grain size. In the case of aluminum alloys, iron, chromium, titanium, or zirconium is advantageously used as the dispersoid-forming element. The weight fraction of the dispersoid-forming element can be between 0.1% and 1% of the material.